Designer Fluorescent Redoxmer Self‐Reports Side Reactions in Nonaqueous Redox Flow Batteries

The state of health (SOH) is a critical measure for evaluating and predicting performance of redox flow batteries (RFBs). However, diagnosing SOH of RFBs is often challenging due to the overwhelming complexity of the electrolytes and associated electrochemical reactions. Designing active molecules or redoxmers that can autonomously exhibit property changes upon specific stimuli may provide a viable way for early diagnosis of SOH. Herein, a dimerized redoxmer, DGL‐N‐CH3, was designed and synthesized by linking blue‐green fluorescent monomers through a diglycolamide linker. While DGL‐N‐CH3 still maintains similar electrochemical behavior and strong fluorescence, we observe a unique side reaction when cycling DGL‐N‐CH3 in H‐cells, which leads to a side product, NHCH3‐BzNSN via linker cleavage. Interestingly, NHCH3‐BzNSN also emits fluorescence but at a longer wavelength. By taking advantage of this unique fluorescent change that corresponds to the growth of NHCH3‐BzNSN, we successfully established the capacity decay of DGL‐N‐CH3 H‐cell cycling, exemplifying a proof‐of‐concept self‐reporting redoxmer design towards in situ SOH monitoring.